A flash flood is the fastest-moving type of flood. It happens when heavy rain collects in a stream or gully, turning the normally calm area into an instant rushing current.

The quick change from calm to raging river is what catches people off guard, making flash floods very dangerous.

Any flood involves water rising and overflowing its normal path. But a flash flood is a specific type of flood that appears and moves quickly across the land, with little warning that it's coming. 

Many things can cause a flash flood. Generally they are the result of heavy rainfall concentrated over one area. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms that repeatedly move over the same area, or heavy rains from hurricanes and tropical storms. 

Dam failures can create the worst flash flood events. When a dam or levee breaks, a gigantic quantity of water is suddenly let loose downstream, destroying anything in its path. 

Flash flood waters move at very fast speeds. They have the power to move boulders, tear out trees, destroy buildings, and obliterate bridges. Walls of water can reach heights of 10 to 20 feet and generally carry a huge amount of debris with them. 

The best response to any signs of flash flooding is to move immediately and quickly to higher ground.